Thursday, 17 November 2016

Sustaining Forests

Thinning treatments

Research Issue

Competition for site resources among trees in a forest is virtually constant. As the crowns and root systems of trees increase in size, severe competition for sunlight, soil nutrients, or soil moisture inhibits the growth of some trees and causes them to die. Tree mortality results in a temporary reduction in competition, thus allowing the surviving trees to capture more site resources and grow to a larger size. As the surviving trees grow, the forest becomes crowded again, and the cycle of competition, mortality, and growth goes on.

Land managers can prescribe thinning treatments to control the degree of crowding in a forest, promote faster growth of desirable species, increase wood yield by removing trees before they die, and influence the diversity and distribution of trees as the forest matures.

To apply effective thinning treatments, land managers need answers to several important questions: How do individual trees respond to reduced crowding? How does thinning affect wood quality and tree vigor? How does thinning affect wildlife habitat? Do harvest operations associated with thinning damage the residual forest? What are the financial impacts associated with thinning?

Our Research

Long-term field trials were employed to test the impact of thinning to various levels of residual forest density. Permanent research sites and the individual trees within them were observed for more than 20 years after commercial, or real-world, thinning operations were conducted. Repeated measurements were collected on more than 18,000 individual trees. Residual density affected overall wood yield, average diameter growth of residual trees, and the dynamics of vertical position, or crown class of individual trees over time. Within those field trials, the impact of free growing space around individual trees was related to tree growth, crown expansion, and wood quality, thus providing insight into the microsite effect of thinning treatments. In a related study, the effect of thinning treatments on the availability of snags for cavity nesting birds provided guidelines for enhancing bird habitat.

Expected Outcomes

Land managers can use this information to prepare thinning treatments that help meet several management objectives in hardwood forests. Specifically, research results define the appropriate level of area-wide residual forest density and the appropriate level of residual crowding around individual trees to maximize growth, vigor, and quality of desirable trees in immature forests. These results also provide an estimate of the effect of thinning on tree crown expansion and crown class dynamics that can be useful in sustaining desired species composition in current and future forests. A major benefit of thinning treatments is that they provide an opportunity to select and favor productive seed trees when the forest is young. Later, those seed trees can contribute abundant seed crops and new seedlings to the regeneration process when the forest is mature.